Our goal is to analyze the molecular mechanisms that underlie the formation of tubulointerstitial (TI) fibrosis, to devise more effective therapies to prevent its progression. We study the collagen receptor integrins ?1?1 and showed that it downregulates collagen synthesis, and its loss leads to increased glomerular fibrosis following injury. Integrin ?1?1 plays an anti-fibrotic action by negatively regulating the phosphorylation state of pro-fibrotic growth factor receptors via activation of the tyrosine phosphatase TCPTP. We have also found that integrin ?1?1 is a negative regulator of TI fibrosis, as integrin ?1KO mice show increased unilateral ureteral obstruction-induced fibrosis. Moreover, integrin ?1KO collecting duct (CD) cells have increased activation of TGF-? receptor (T?R)-dependent pro-fibrotic signaling, such as phosphorylated Smad3 and collagen levels. TGF-? exerts its functions via activation of the serine/threonine kinases T?RI and T?RII. Binding of TGF-? to T?RII leads to phosphorylation of T?RI and subsequent activation of the major pro-fibrotic mediator Smad3. Interestingly, the cytoplasmic tail of T?RII can also be phosphorylated on tyrosine residues. However, whether these tyrosines play any physiological or pathological role in renal cells is unknown. The novelty of this proposal is that integrin ?1KO CD cells show increased basal levels of tyrosine phosphorylated T?RII. This result suggests that, in renal cells, integrin ?1?1 crosstalks with T?RII and it might prevent its pro-fibrotic action by downregulating its tyrosine phosphorylation levels. Interestingly, inhibition of TCPTP in CD cells leads to increased Smad3 activation and collagen synthesis. This result, together with the finding that 3 tyrosines in the T?RII tail can be potential substrates of TCPTP, forms the hypothesis that integrin ?1?1 negatively regulates T?RII tyrosine phosphorylation via activation of TCPTP. Thus, we propose that integrin ?1?1/TCPTP-mediated dephosphorylation of T?RII represents an important, but previously undescribed mechanism to selectively reduce T?RII activation and consequent progression of fibrosis. To test this hypothesis: Aim 1 will analyze the role of T?RII-mediated pro-fibrotic signaling in TI injury in the integrin ?1KO mice. We will cross integrin ?1KO mice with global null or floxed Smad3 mice to determine if preventing T?R/Smad3 axis in the collecting system is sufficient to ameliorate TI fibrosis in the ?1KO mice. We will then determine if in vivo activation of TCPTP is beneficial in the setting of TI injury by counteracting T?R-mediated pro-fibrotic action. Aim 2 will determine the mechanisms whereby integrin ?1?1 negatively regulates T?RII. We will use in vitro approaches to analyze i) if TCPTP directly binds and dephosphorylates T?RII; ii) if tyrosine residues are important to control T?RII-mediated Smad3 activation and collagen synthesis; and iii) which tyrosine(s) controls T?RII-mediated functions. This study will lead to the identification of a novel crosstalk between integrin ?1?1 an T?RII and, most importantly, a novel mechanism whereby T?RII-mediated pro-fibrotic signaling can be negatively modulated.